1. Field of the Invention
The present invention relates to radio IC devices, and more particularly, to a radio IC device preferably for use in a radio-frequency identification (RFID) system for noncontact data communication using electromagnetic waves.
2. Description of the Related Art
Recently, RFID systems for transmitting information by noncontact communication between a reader/writer that generates an induction field and an RFID tag attached to an article and storing predetermined information have been used as article management systems.
Japanese Unexamined Patent Application Publication No. 2005-252853 discloses an RFID antenna that transmits and receives frequencies used in RFID, such as 13.56 MHz, 950 MHz, and 2.45 GHz.
FIG. 1 is a diagram showing the configuration of a noncontact IC tag that includes the RFID antenna of Japanese Unexamined Patent Application Publication No. 2005-252853. As shown in FIG. 1, antenna portions 111, 112, and 113 and land portions 103a and 103b are formed on a base substrate 102 made of, for example, resin or paper. The antenna portion 111 is formed of a coil of conductive material. The antenna portion 112 is formed of two L-shaped conductors 112a and 112b arranged at a predetermined distance from each other so as to surround the antenna portion 111. The antenna portion 113 is formed of two conductors 113a and 113b arranged at a predetermined distance outside the antenna portion 112. The land portions 103a and 103b are connected to the antenna portions 111 to 113. An IC chip 101 is mounted on the land portions 103a and 103b. 
However, an RFID antenna having the structure as shown in Japanese Unexamined Patent Application Publication No. 2005-252853, has a problem in that the antenna size is relatively large because three different antennas must be provided on the same substrate. Another problem is that, if a plurality of antennas are disposed in close proximity and one of them is used for radio communication, the other antennas interfere with the communication, thus leading to unstable communication.